Sensor and method for measuring a current of charged particles

ABSTRACT

A current sensor ( 1 ) is disclosed for measuring a magnetic field ( 8 ) induced by a current of charged particles ( 3 ) having at least one magneto resistive sensor element ( 2;6;12;16 ) for enclosing the magnetic field induced by the current of charged particles, the magneto resistive sensor element being arranged perpendicularly to the current ( 3 ) during use. A method for accurately determining a current of charged particles is also disclosed making use of the current sensor ( 1 ). Further a protective switch device ( 30 ) is disclosed for protecting a user of an electrical device ( 31 ) by switching a supply current to the electric device off in case of malfunction of the electric device is also provided comprising the above current sensor ( 1 ).

The invention relates to a sensor for measuring a magnetic field inducedby a current of charged particles.

The invention further relates to a method for measuring a current ofcharged particles using the inventive sensor.

The invention further relates to a protective switch device in which theinventive sensor and method are used.

A beam of charged particles induces a magnetic field outside the beam,which field may be measured by a current sensor for measuring a magneticfield. By measuring this field using a magnetic sensor, e.g. a sensorbased on the Hall effect or a sensor based on the tunnelingmagnetoresistance (TMR) or a sensor based on the anisotropicmagnetoresistance (AMR) effect, known from “The magneto resistivesensor”, Tech. Publ. 268, Philips Electronic Components and Materials,or a sensor based on the giant magneto resistance effect (GMR), see“Robust giant magneto resistance sensors”, K.-M. H. Lenssen, D. J.Adelerhof, H. J. Gassen, A. E. T. Kuiper, G. H. J. Somers and J. B. A.D. van Zon, Sensors&Actuators A85, 1 (2000), the current can bedetermined in a “non-intrusive” way.

The magnitude of the field H as a function of the distance from thecenter of the current I is given by: $\begin{matrix}{H = \frac{1}{2\pi\quad r}} & (1)\end{matrix}$, assuming a circular cross section of the current, see FIG. 1. Thesensor can be implemented by a current clamp, which is only clampedaround the conductor for measurement, or can be included in a chipcomprising also the current-carrying conductor. Current-sensor chipsare, for example, known from U.S. Pat. No. 5,621,377, in which AMRelements on top of a conductor are used to measure the current in thisconductor in a “contactiess” way.

A limitation of all present current sensors is the sensitivity toexternal disturbing fields. Most of these sensors rely on themeasurement of the magnetic field at only one point outside the currentcarrying conductor. Only if the distance between sensor and current isexactly known and if there are no disturbing magnetic fields, a correctdetermination of the current amplitude can be made. In practice,however, there are always other magnetic fields present, like e.g. theearth magnetic field.

Current clamps indeed “average” the magnetic field over a certain lineby means of a soft-magnetic yoke, but still disturbing fields enteringthrough the non-magnetic gap in which the sensor is placed usually limitthe performance. Moreover, the current-clamp geometry is less favorablefor magneto resistive sensors than for Hall-sensors, since the latter issensitive for perpendicular fields; however, the sensitivity ofHall-sensors is much lower.

One has tried to mitigate this problem by using a multitude ofHall-sensors to measure the magnetic field at several positions outsidethe conductor, see V. V. Serkov, “Contactless dc ammeters”, Pribory iTeldnika Éksperimenta 5, pp. 169-171, 1991. However, this configurationand the required electronics is complex and expensive, and the currentmeasurement is still principally not correct, since theoretically onehas to measure the loop integralc∫{right arrow over (H)}·{right arrow over (d)}l=I _(enclosed).  (2)Further, the above-mentioned problems have so far hindered therealization of a “residual-current switch” that is suitable for use inconsumer electronics, e.g. hair dryers, although there is a seriousdemand and potentially enormous market for such a device. The sensor insuch a device should be able to detect a difference of 2 or 10 mA oncurrents with an amplitude of up to 16 A and should contain no bulkyparts, as is the case in the residual-current switches used in houses.

Therefore the invention has for its object to provide a sensor and amethod for measuring currents of charged particles more accurately andbeing intrinsically insensitive to external disturbing magnetic fields.

To achieve the object, the sensor for measuring a magnetic field inducedby a current of charged particles according to the invention comprisesat least one magneto resistive sensor element for enclosing the magneticfield induced by the current of charged particles, the magneto resistivesensor element being arranged perpendicularly to the current during use.

In order to determine a current exactly, one has to measure theabove-mentioned integral equation (2) along a path surrounding thecurrent of charged particles. While this is practically impossible toachieve by most sensor types, a unique characteristic of magnetoresistive sensors (TMR, AMR or GMR) can be exploited for this purpose.With a suitable configuration of the sensor elements, the magnetic fieldis “automatically” integrated along the sensor. The current of chargedparticles can be e.g. a current of electrons, holes or ions.

The resistance R of such a magneto resistance element, being forinstance a strip, is given by:R=∫ρdl=∫(ρ _(o)+Δρ)dl=R _(o) +∫Δρdl.  (3)Since the equation:Δρdl∝{right arrow over (H)}·{right arrow over (d)}l=I _(enclosed),  (4)is valid, a current sensor can be realized based on the fundamentalprinciple of equation (2). Because the above integral along a closedloop can be determined in the sensor of the invention, insensitivity todisturbing, external fields is achieved. The directional sensitivityinherent to the magneto resistive effect automatically yields therequired inproduct at least as long as the sensor is perpendicular tothe plane of the current of charged particles. External fields have noinfluence at all on the measurement outcome, and moreover the shape ofthe path and the position of the current of charged particles within theloop are of no importance. An additional advantage of the sensoraccording to the invention is that since the integration is built-in inthe sensor, additional electronic circuits can be simplified.

According to a preferred embodiment of the invention, the magnetoresistive sensor element has a circular shape. This preferred embodimenthas the advantage that the circumference of the circle is well definedwhich makes the integration along the loop easy. Moreover, manufacturingof such a circular shape is relatively easy.

The magneto resistive sensor element is therefore preferably made on aflexible substrate. This feature enables to wrap the magneto resistivesensor element around the current of charged particles in order tomeasure the magnetic field. The charged particles can be electrons,flowing for instance in a conductor. If the magneto resistive elementencloses the magnetic field of the conductor, external fields will haveno influence at all on the measurement outcome. Moreover the shape ofthe path and the position of the conductor or a plurality of conductorswithin the loop of the magneto resistive sensor element is of noimportance.

According to a preferred embodiment of the invention, the magnetoresistive sensor element is a strip. The resistance of such a strip ofmagneto resistive material is well defined, the specific resistancebeing p. According to equation (3) and (4) the current of chargedparticles can be determined. Usually a multi-layer structure ofmaterials is used.

It is an advantage that the sensor can be made in thin film technology.This advantageous feature enables the production of very small and verylight elements which can be used for domestic appliances.

Preferably, the magneto resistive sensor element has a linear resistanceversus magnetic field R(H) characteristic. This enables to determine themagnetic field of the current exactly.

In order to compensate for temperature effects, preferably the sensorelements are arranged in a Wheatstone bridge configuration. TheWheatstone bridge circuit enables the temperature compensatedmeasurement of the magnetic field.

According to a preferred embodiment of the invention, two magnetoresistive sensor elements of the Wheatstone bridge configuration arepresent on one side of the flexible substrate and the other two magnetoresistive sensor elements are present on the other side of the flexiblesubstrate. The two magnetoresistive elements are usually a strip and arearranged parallel to each other.

During or after deposition of the multi-layer structure, themagnetization direction of a pinned layer in the multi-layer structurecan be set by applying a magnetic field. The two magneto resistiveelements on one side of the flexible substrate get the samemagnetization direction. The flexible substrate is subsequently turned,and an identical multi-layer is deposited on the other side of theflexible substrate, getting an opposite magnetization direction.

Preferably a pair of two magneto resistive sensor elements of theWheatstone bridge configuration has been stacked on top of the otherpair of magneto resistive sensor elements, and between the two pairs aninsulating material is present and a conductor is present for carryingthe current of charged particles. The sensor is made in thin filmtechnology and is therefore very suitable to be integrated on an IC.Since the current sensor can measure small currents very accurately, thesensor is very useful in for instance a magnetic memory, e.g. toaccurately measure the read or write current.

To achieve the object of the invention a method for measuring a currentof charged particles using the sensor as described here above,comprising the steps of:

-   -   determining a change in resistance in the sensor according to        the invention caused by a magnetic field induced by the current        of charged particles,    -   comparing the change in resistance with a reference        characteristic of the sensor of the resistance versus magnetic        field and determining the magnitude of the magnetic field,    -   calculating the magnitude of the current from the magnitude of        the magnetic field.

An additional advantage of the sensor according to the invention is thatsince the integration is built-in in the sensor, the electronic circuitcan be simplified. The known R(H) curve of the magnetoresistive sensorelement can be used as a reference in a comparator circuit. A linearR(H) curve allows exact determination of the magnetic field value fromthe change in resistance. If the magnetorsistive sensor elements arearranged in the Wheatstone bridge configuration and the magnetoresistivesensor elements have a circular shape in the form of a strip, theenclosed current of charged particle follows from the product of the Hvalue and the circumference of the magnetoresistive sensor elements.

For accurately measuring a residual current, the sensor with a conductorin between the two pairs of magneto resistive elements in a Wheatstonebridge configuration can be used. A current is sent through a firstconductor and a current having an opposite sign is sent through a secondconductor positioned parallel to the first conductor. Such a principleis useful in a residual current switch.

To achieve the object of the invention a protective switch forprotecting a user of an electrical device by switching a supply currentto the electric device off in case of malfunction of the electricdevice, comprising a sensor as described here above, and furthercomprising:

-   -   a comparator circuit comparing an output current or voltage of        the current sensor with a reference current or voltage        respectively, and    -   a relay device switching the supply current dependent on the        output current or voltage of the comparator circuit. The        protective switch device is suitable for integration in domestic        appliances for example in a hairdryer, because it is small and        light and has no bulky elements.

The output signal of the compare circuit can be connected to a relaywhich opens at least one switch and stops the current flow when thedetermined difference between the currents flowing in the conductors istoo high.

These and various other advantages and features of novelty whichcharacterize the present invention are pointed out with particularity inthe claims annexed hereto and forming a part hereof However, for abetter understanding of the invention, its advantages, and the objectobtained by its use, reference should be made to the drawings which forma further part hereof, and to the accompanying descriptive matter inwhich there are illustrated and described preferred embodiments of thepresent invention.

FIG. 1 shows the magnetic field surrounding a current;

FIG. 2 a shows a side view of strip-like sensor elements fabricated on aflexible substrate;

FIG. 2 b shows a cross sectional view of the strip-like sensor elementsalong the line 11-11 in FIG. 2 a;

FIG. 3 shows an equivalent circuit diagram of the magneto resistivesensor elements connected in a Wheatstone bridge configuration;

FIG. 4 shows the output characteristic of the magneto resistive sensorelements connected in a Wheatstone bridge configuration;

FIG. 5 shows a thin film embodiment of the magneto resistive sensorelements measuring the magnetic field of one conductor;

FIG. 6 shows a thin film embodiment of the magneto resistive sensorelements measuring the magnetic field of two conductors with oppositecurrent directions; and

FIG. 7 shows a block diagram of a protective switch device forprotecting users of electrical devices.

FIG. 1 shows a magnetic field of a current I. The amplitude of themagnetic field H decreases when the distance r to the current I flowingin a conductor is increased. The length of the arrows in FIG. 1characterize the amplitude of the magnetic field H. The stronger themagnetic field is, the longer is the arrow. The drawn circles show thelines of equal amplitude of the magnetic field H. By measuring themagnetic field H, the current I flowing in a conductor can bedetermined. The magnetic field H is connected to the current I and thedistance r by the equation 1.

FIG. 2 a shows a side view of a magneto resistive sensor 1 and FIG. 2 bshows a cross sectional view of the magneto resistive sensor 1 takenalong the line II-II in FIG. 2 a. In this embodiment the sensorcomprises four magneto resistive elements (2,12,6,16). The side view ofFIG. 2 a shows the conductor 10 through which a current of chargedparticles 3 flows. Two magneto resistive sensor elements 2 and 12 areprovided on an insulating flexible substrate 4, for instance a foil. Themagneto resistive sensor elements 2, 12 are fabricated at the same time,e.g during the same sputter deposition process. The magnetizationdirection 5 of the magneto resistive sensor elements 2 and 12 isidentical. The magneto resistive sensor elements 2,12 can be insulatedfrom each other by an electrically insulating material, e.g siliconoxide, and can be covered with a protection layer.

The arrows drawn on the magneto resistive sensor elements 2 and 12 showthe biasing direction when four magneto resistive sensor elements areconnected in a Wheatstone bridge circuit configuration. The Wheatstonebridge circuit compensates the measurements from temperature influence.The arrows in FIG. 2 a show the biasing directions of the magnetoresistive sensor elements 2 and 12, which are arranged on top of theother two magneto resistive sensor elements 6 and 16. It is to be notedthat the biasing directions of the magneto resistive sensor elements 2,12 are opposite to the magneto resistive sensor elements 6, 16.

In the cross sectional view of FIG. 2 b, the magneto resistive sensorelement 2 is present on top of the insulating flexible substrate 4. Onthe other side of the flexible substrate 4 a strip-like sensor elements6 is present. In depth, the magneto resistive sensor elements 12, 16 arepresent. The conductor 10 is located in the center of the crosssectional view. The current I flowing in the conductor 10 generates themagnetic field 8. In order to show the principle only one line of themagnetic field 8 is drawn. The magnetic field 8 is measured by themagneto resistive sensor elements 2,6,12,16. In this embodiment themagneto resistive sensor has a circular shape, but the shape of thesensor is not limited thereto and can be for example squared orrectangular.

The strip-like sensor elements 2,6,12,16 may comprise a GMR multi-layere.g. an exchange biased spin valve with its exchange-biasing directionalong the strip direction. A spin valve structure based on the GMReffect can be manufactured as follows: On a insulating substrate 4 amulti-layer structure is deposited of a buffer laag of 3.5 nm Ta/2.0 nmPy to induce the right (111) texture,

-   -   a magnetic layer having a magnetization axis 5 being the pinned        layer, comprising an exchange biasing layer of 10 nm Ir₁₉Mn₈₁        and an artificial antiferromagnet of 3.5 nm Co₉₀Fe₁₀/0.8 nm        Ru/3.0 nm Co₉₀Fe₁₀,    -   a non-magnetic spacer layer of 3 nm Cu, and    -   a ferromagnetic layer of 5.0 nm Py: the free layer (with below        e.g. a thin layer of 1.0 nm Co₉₀Fe₁₀ which enhances the GMR        effect and reduces the interlayer diffusion by which the thermal        stability is increased). A protection layer of 10 nm Ta is        deposited on top of the multi-layer.

Alternatively the magnetoresistive element can be a magnetic tunneljunction comprising the following multilayer-structure: a buffer layerof 3.5 nm Ta/2.0 nm NiFe, an exchange biasing layer and a pinned layer(AAF) being the magnetic layer: 15.0 nm IrMn/4.0 nm CoFe/0.8 nm Ru/4.0nm CoFe, a non-magnetic spacer layer of 2.0 nm Al₂O₃, and a secondferromagnetic layer of e.g. 6.0 nm CoFe: the free layer.

The magnetization direction of the pinned layer of the GMR multilayerhas been applied during sputter deposition in a magnetic field. Themagneto resistive sensor elements 2,12 and 6,16 have been fabricatedafter each other in different sputter deposition processes. Themagnetization direction 5 of the magneto resistive sensor elements 2,12and 6,16 are opposite to each other. The arrows in FIG. 2 b indicate themagnetization direction 5 of the pinned layer in the sensor elements 2and 6 on both sides of the insulating flexible substrate 4.

In order to determine a current exactly, one has to measure theabove-mentioned integral equation (2) along a path 8 surrounding thecurrent conductor 10. If this can be obtained, external fields have noinfluence at all on the measurement outcome, and moreover the shape ofthe path and the position of the conductor within in the loop is of noimportance.

A unique characteristic of magneto resistive sensors (TMR, AMR or GMR)can be exploited for this purpose: if a suitable configuration ischosen, the magnetic field is “automatically” integrated along thesensor.

The resistance R of such an magneto resistance strip is given by:R=∫ρdl=∫(ρ_(o)+Δρ)dl=R _(o) +∫Δρdl.  (3)Since the equation:Δρdl∝{right arrow over (H)}·{right arrow over (d)}l=I _(enclosed)  (4)is valid, a current probe can be realized based on the fundamentalprinciple of equation (2). As this integral along a closed loop can bedetermined in the embodiments of the invention, insensitivity todisturbing, external fields is provided. Since the integration isbuilt-in in the sensor, the electronics can be simplified. Thedirectional sensitivity inherent to the magneto resistive effectautomatically yields the required inproduct at least as long as thesensor is perpendicular to the plane of the conductor cross section.Moreover, all elements are now continuous, i.e. there are no gapsbetween the sensor parts except for a small gap for the electricalcontacts.

FIG. 3 shows an equivalent circuit diagram of the magneto-resistivesensor elements connected in a Wheatstone measurement bridgearrangement. The measurement bridge comprises four magneto-resistivesensor elements 2, 12, 6, 16. The two magneto-resistive sensor elements6 and 12 are connected to a first terminal 20 of the bridge. The firstterminal 20 is the input terminal of the sense current currentI_(sense). The magneto resistive sensor element 12 is connected to asecond or measurement terminal 24 of the bridge. The magneto resistivesensor element 6 is connected to a third or measurement terminal 26. Themagneto-resistive sensor elements 2 and 16 are connected to a forth oroutput terminal 22 of the bridge where the output current is present. Onthe other side is the magneto resistive sensor element 2 connected tothe measurement terminal 26 where the measurement voltage is present.The magneto resistive element 16 is connected to the measurementterminal 24.

The voltage is measured between the terminals 24 and 26 in order todetermine a voltage value characterizing the measured magnetic field H.The advantage of the Wheatstone measurement bridge is that itcompensates the influence of the temperature on the measurement value.For magnetic field sensors it is often desirable to eliminate theinfluence of temperature variations and to realize a bipolar output bythe use of a Wheatstone measurement bridge configuration. Themagneto-resistive sensor elements in two of the bridge branches shouldhave an opposite response to a magnetic field than the other twoelements, as shown in FIG. 3 by the direction of the arrows. The arrowsdemonstrate the direction of the magnetic basing direction of themagneto-resistive sensor elements. In the case of AMR elements, theopposite response can be achieved by placing the magnetic biasingdirections under −45 and +45° on the two pairs of the magneto-resistivesensor elements.

FIG. 4 shows the output voltage of the GMR-Wheatstone bridgeconfiguration of the embodiment of FIG. 2. At a bias voltage of 5V(corresponding to a sense current of 2.5 mA and a resistance of thebridge of 2 kOhm), the sensor has a linear output characteristic forsmall magnetic fields over a large temperature range between 20-200° C.Small magnetic fields can be accurately measured. The GMR effect is 6%,with a small hysteresis and a very small offset voltage drift of 0.7μV/K.

From the linear output characteristic 13 of the magnetoresistive sensorelements in the Wheatstone bridge configuration, the value of themagnetic field is determined.

For the circular shape of the magnetoresistive sensor elements, thecurrent enclosed follows from the value of the magnetic field times 2πr.

FIG. 5 shows a thin film embodiment of the magneto-resistive sensorelement measuring the magnetic field of one conductor. The sensorelements 2,12 and 6,16 are stacked on top of each other. Only two sensorelements 2,6 are shown. The sensor elements 2,12 have been separatedfrom the sensor elements 6,16 by an electrically insulating material 7.For consumer electronics it may be desirable to have a thin film device.In this case a continuous surrounding of the conductor cannot beachieved in a practical way, but it can be approximated well by usingtwo magneto-resistive elements.

The embodiment of FIG. 5 comprises four magneto-resistive sensorelements 2,12 and 6,16 in a Wheatstone bridge configuration, anon-magnetic wire 15, a current carrying conductor 10 and an insulatingmaterial 7. The magneto resistive sensor elements 2,6 of thehalve-bridge are connected in series. The magneto resistive sensorelements 2 and 6 have opposite biasing directions and are electricallyconnected in series by means of a non-magnetic wire 15, e.g. a metallike Al or Cu. If the length of the magneto-resistive sensor elements 2and 6 is significantly longer than the distance between them and if theedges are relatively far away from the conductor 10, the serialresistance of the two magneto resistive sensor elements 2 and 6 will bea very good measure for the current through the conductor. If desired,special shaped ends could be added to the elements in order to reducethe non-magneto resistive gaps.

FIG. 6 shows a thin film embodiment of the sensor measuring the magneticfield of the two conductors 10,11 with opposite current directions. Theembodiment of FIG. 5 comprises four magneto-resistive sensor elements2,12 and 6,16 in a Wheatstone bridge configuration, a non magnetic wire15, two current carrying conductors 10 and 11 with opposite currentdirections and an insulating material 7. The two magneto-resistivesensor elements 2 and 6 of the halve-bridge are connected in series bythe non-magnetic wire 15. The difference between the embodiment of FIG.6 to the embodiment of FIG. 5 is that the embodiment of FIG. 6 measuresthe difference of the two magnetic fields of the two current carryingconductors 10 and 11. The high sensitivity of the embodiment of FIG. 6makes it very suitable for application in a residual current switch. Ifthe two currents with opposite directions are both enclosed by thesensor loop, the summation of their accompanying magnetic fieldsautomatically results in a measurement of the difference between bothcurrents. This also helps to avoid saturation of the magneto resistiveembodiment. If both currents are equal, but opposite, the sensor outputwill be zero; if a difference arises a non-zero output will result. Incontrast to inductive sensors, magneto resistive sensor elements canalso be used for dc currents.

FIG. 7 shows a block diagram of a protective switch device 30 forprotecting a user of an electrical device. The block diagram comprisestwo terminals 34 and 35 for an electric power supply. The terminal 34 isswitched by a switch 36. The terminal 35 is switched by a switch 37. Thetwo switches 36 and 37 are switched in parallel by a relay 33. The twoswitches 36 and 37 are connected on the other side to a load 31 beingfor example a motor.

Sensor 1 measures a difference of the two currents flowing to and fromthe load. The two terminals 20 and 22 supply a small sense current forthe sensor 1. The sense current is the input current for the sensor 1needed to measure its resistance. The output signal of the sensor 1supplied by two terminals 24 and 26 goes to a comparator circuit 32. Thecomparator circuit 32 compares the output of the magneto-resistivecurrent sensor 1 with a threshold value provided by terminal 38. In caseof malfunction the current sensor 1 determines a difference between thetwo currents and gives an output signal to the comparator circuit 32.The comparator circuit 32 compares the output with a reference value 38.In case of malfunction, the comparator circuit 32 gives an output signalto the relay 33 in order to open the two switches 36 and 37. The blockdiagram of a protective switch device can be applied for instance in ahair dryer or in a circuit for detecting the on-state of head lights incars where a missing current flow would indicate that the head light isbroken.

The current sensor of the above embodiments of the invention areapplicable in many different environments, for example for measuring themagnetic field of single conductors, cables, conductor paths inintegrated circuits and the electric current presented by a beam ofcharged particles, like electrons or ions. Measuring of the magneticfield of conductors paths in integrated circuits could be integratedinto on-chip testing techniques for testing, for example, currentcontacts.

New characteristics and advantages of the invention covered by thisdocument have been set forth in the foregoing description. It will beunderstood, however, that this disclosure is, in many respects, onlyillustrative. Changes may be made in details, particularly in matters ofshape, size, and arrangement of parts, without exceeding the scope ofthe invention. The scope of the invention is, of course, defined in thelanguage in which the appended claims are expressed.

1. A sensor for measuring a magnetic field induced by a current ofcharged particles comprising at least one magneto resistive sensorelement for enclosing the magnetic field induced by the current ofcharged particles, the magneto resistive sensor element being arrangedperpendicularly to the current during use.
 2. The sensor as claimed inclaim 1, wherein the magneto resistive sensor element has a circularshape.
 3. The sensor as claimed in claim 1 or 2, wherein the magnetoresistive sensor element is present on a flexible substrate.
 4. Thesensor as claimed in claim 1, 2 or 3, wherein the magneto resistivesensor element is a strip.
 5. Sensor as claimed in claim 1, wherein themagneto resistive sensor element has a linear R(H) characteristic. 6.The sensor as claimed in anyone of the claims 1 to 5, wherein magnetoresistive sensor elements are arranged in a Wheatstone bridgeconfiguration.
 7. The sensor as claimed in claim 6, wherein two magnetoresistive sensor elements of the Wheatstone bridge configuration arepresent on one side of the flexible substrate and the other two magnetoresistive sensor elements are present on the other side of the flexiblesubstrate.
 8. The sensor as claimed in claim 7, wherein the two magnetoresistive elements on one side of the flexible substrate have the samemagnetization direction.
 9. The sensor as claimed in claim 6, wherein apair of two magneto resistive sensor elements of the Wheatstone bridgeconfiguration has been stacked on top of the other pair of magnetoresistive sensor elements, and between the two pairs an insulatingmaterial is present and a conductor is present for carrying the currentof charged particles.
 10. Method for measuring a current of chargedparticles using the sensor as claimed in anyone of the claims 1 to 9,comprising the steps of: determining a change in resistance in thesensor according to the invention caused by a magnetic field induced bythe current of charged particles, comparing the change in resistancewith a reference characteristic of the sensor of the resistance versusmagnetic field and determining the magnitude of the magnetic field,calculating the magnitude of the current from the magnitude of themagnetic field.
 11. Method as claimed in claimed 10, making use of thesensor according to claim 9, wherein a current is sent through a firstconductor and a current having an opposite sign is sent through a secondconductor positioned parallel to the first conductor for measuring aresidual current.
 12. A protective switch device for protecting a userof an electrical device by switching a supply current to the electricdevice off in case of malfunction of the electric device, comprising asensor as claimed in any of the claims 1 to 9, and further comprising: acomparator circuit comparing an output current or voltage of the currentsensor with a reference current or voltage respectively, and a relaydevice switching the supply current dependent on the output current orvoltage of the comparator circuit.